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Why I don’t like IEntity interfaces

One comment that keeps popping up in regards to my YouTube video about repositories (Repository Pattern, Done Right) is why I don’t like IEntity interfaces. So, let’s see.

The case for IEntity is to apply a constraint in the repository interfaces.

public interface IRepository<T> where T : IEntity 

I need to write another post about why you should avoid generic repositories. But for now, let’s just ignore it and focus on IEntity.

So, what’s the value of this constraint here? Does this force a developer from accidentally creating a repository for an integer?

public class IntRepository : IRepository<int>

Probably not! Those familiar with domain-driven design (DDD) argue that repositories are should be created for aggregate roots. So, this will stop a developer from creating a repository for an entity that is not an aggregate root, or a value object. So, perhaps it’s more accurate to call this interface IAggregateRoot (which is even worse than IEntity, as you’ll find out shortly).

With either IEntity or IAggregateRoot, a junior developer can bypass this “constraint” in just a second:

public class Money : IEntity
{
}

So, these marker interfaces don’t really stop anyone from creating a repository for the wrong type!

With these interfaces, every time you want to create a class, you need to write some additional code (noise) to mark it as an entity or aggregate. What is the value of this? I’ve never seen it! Hopefully, you can drop a comment and enlighten me.

What is an interface really?

An interface is a contract. When you declare an interface and implement it, all those implementations follow that contract. This gives you two benefits: extensibility and testability.

Think of the classic polymorphism example you read in your first object-oriented book.

IEnumerable<IShape> shapes = GetShapes();
foreach (var shape in shapes)
    shape.Draw(); 

Here, the type of the actual objects in our enumerable doesn’t really matter. We can “extend” this design, and define new shapes that implement this contract (IShape). At a minimum, they all will have a Draw method.

Another benefit of these contracts is that they help us mock dependencies of objects during unit testing. For example, we can unit test a controller or a service by providing a mock or fake repository. This way, we don’t need a database up and running to test the controller / service.

The case for IEntity

Look at this IEntity declaration. This is what I call a hollow interface.

public interface IEntity // or IAggregateRoot
{
}

What is the point of this interface? It’s an empty contract. What’s that supposed to mean “conceptually”? A contract that doesn’t enforce anything?! Really? Again, back to the first example, if you think by using these interfaces you prevent an inexperienced developer creating the wrong repository, you’re just fooling yourself.

Wait, I have a better solution!

Ok, Mosh, how about this interface?

public interface IEntity // or IAggregateRoot
{
    int Id { get; set; }
}

Not a hollow interface anymore! At least it enforces that every entity or aggregate root should have an Id, and that Id should be integer.

Wait… what if one of our entities needs a Guid as an Id?

Ok, what about IEntity<T>?

Here is another form of IEntity:

public interface IEntity<T> // or IAggregateRoot<T>
{
    T Id { get; set; }
}

This interface is even worse. Why? Look at how we use it:

public class Student : IEntity<int>

Read the code in plain English: class Student which is an entity of integer. What?! Entity of integer? (*my head spinning*).

Nonetheless, what value do you get from an interface like that? That every entity or aggregate root follows a contract? Would you be working with those IEntity or IAggregateRoot instances without caring what object is actually under the interface at runtime (like the polymorphism example I showed earlier)? Most likely not! You work with concrete types, like Student, Course, etc.

So, one more time, what’s the point of creating these contracts? To the best of my knowledge, nothing but extra noise in your code.

Unfortunately, interfaces are one of the least understood constructs of object-oriented programming languages. If you want to learn more about interfaces, check out my C# course: Classes, Interfaces and Object-0riented programming.

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Repositories or Command / Query Objects?

This is a follow up post to my YouTube video Repository Pattern, Done Right. If you haven’t seen this video, it’s best to watch it first, before continuing reading.

There have been many interesting questions in the discussion area and I’ve replied to most (if not all) of them. One that has come up a few times is:

Should we favour command/query objects over repositories?

A few have referred me to Jimmy Bogard’s post. So, in this post, I’ll be comparing these two approaches and pros / cons of each.

Before I get started, I need to emphasise: neither the repository pattern, nor command / query objects, are silver-bullets. Just because these architectural patterns exist, doesn’t mean you should use them in every single application. If your queries are simple enough (one or two lines), don’t worry about creating an abstraction over them. There is nothing wrong with writing them in your controllers (assuming a web application).

If you do, however, have fat queries that have bloated your controllers, you might want to consider one of these patterns to achieve “better separation of concerns”.

Repository vs Command / Query object

In a repository, we often have many methods, all related to a specific entity:

GetUpcomingGigs()
GetGigsIAmAttending()
GetMyGigs()
GetPopularGigs()

When you promote one of these methods to a class, you’ll end up with what we call a query object.


public class GetPopularGigsQuery
{
     private GigsContext _context;

     public GetPopularGigsQuery(GigsContext context) 
     {
          _context = context;
     }

     public IEnumerable<Gig> Execute() 
     {
          return _context.Gigs
                      .Where(...)
                      .Include(...)
                      .Include(...)
                      .Include(...)
                      .OrderByDescending(...)
                      .Take(10)
                      .ToList();
     }
}

So, what you see in the Execute() method here, used to be one of the methods of a class like GigsRepository. Now, we’ve promoted that to a class. Have we gained anything here? Absolutely not!

With this approach, if you have a repository with 5 methods, you’ll end up with 5 new classes in your application. Now, take into account how many repository classes you have and eventually how many command / query classes you’ll end up with. You’ll end up with an explosion of classes that don’t provide any additional value than a simple method.

When Command / Query objects provide value

When you want to execute some operation before and / or after the execution of the command / query. Sometimes, in large enterprise applications, you may want to check that the current user has permission to execute a given query (or command), and additionally you may want to log that somewhere.

Without using command / query classes, your repository methods would end up with a lot of duplicate logic:

public IEnumerable<Gig> GetPopularGigs()
{
     // authorisation logic 

     // logging logic 
     
     return _context.Gigs.......
}

public IEnumerable<Gig> GetUpcomingGigs()
{
     // authorisation logic 

     // logging logic 
     
     return _context.Gigs.......
}

At this point, by promoting a method to a class, you’ll have at least a couple of options in front of you to implement the same behaviour in a more elegant way. You can use the template or strategy pattern.

With the template pattern, you would define a base class for your commands or queries like this:

public abstract class Query 
{
      public void Execute()
      {
           // authorisation logic 

           // logging logic
 
           DoExecute();
      }

      abstract void DoExecute();
}

So, you write the authorization and logging logic only once, in the base Query or Command class. Then, when implementing an actual command or query, you override the DoExecute method:

public abstract class GetPopularGigsQuery : Query 
{
      public override DoExecute()
      {
           return _context.Gigs....
      }
}

With the strategy pattern, instead of using inheritance, you’d use composition to achieve the same thing. If you need more understanding of inheritance vs composition, check out my C# course: Classes, Interfaces and Object-oriented programming.

What about ASP.NET MVC action filters?

Mosh, aren’t action filters exactly for that? To execute some custom logic (e.g. like authorisation or logging) before each action? True! Then, why bother using the command / query pattern?

Good question! First, you don’t have to use the pattern, if you prefer to use ASP.NET MVC action filters. But what if, in the future, you decide to use a different web presentation framework that doesn’t have the concept of MVC filters? Tools and frameworks come and go. What is popular now, is not going to be popular in a few years time. There was a time we thought ASP.NET WebForms was so cool. Now, it’s almost a dead technology!

If you want to protect your application architecture from change of external tools and framework, you need to decouple it from them by creating your own abstractions. Command / query pattern is one of the ways to create such abstraction. If you switch over to a different presentation framework, you can take all your existing code and re-use it without modifying a single line, and more importantly, without breaking things along the way!

What if…

What if you’re not too concerned about decoupling your application architecture from external tools / libraries? What if you think this is overkill? That’s a perfectly valid argument. Then, don’t use these patterns. As I always say to my students: “Keep it simple”. Use MVC filters to achieve the same result, and if they won’t help you in the future, then you’d spend time solving that new problem, not now.

What if you don’t have such requirements like authorisation and logging before each query? Then, command / query objects don’t give you any additional value. You’ll just waste your time and energy writing more and more code.

What if you don’t have complex queries in the first place? Don’t bother with the repository pattern either!

Every pattern is designed to solve a specific problem. Don’t solve a problem that does not exist!

Till my next post, all the best my friends!

 

 

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Top 3 Reads for C# Developers

I’ve had quite a few students of mine contacting me and asking:

Mosh, apart from your video courses, what books should I read to become a better developer?

If you want to become a kick-ass C# developer, you need to master the fundamentals. By fundamentals, I mean concepts that are always applicable. ASP.NET 5 is NOT one of them! Neither are Entity Framework, Xamarin, Azure, etc.

Frameworks and tools come and go. Becoming proficient in a framework may give you some career opportunities in the short term, but if you want to be ahead of the game, you need to understand the core principles. When you understand the fundamentals well, you can quickly learn a new framework or a new programming language. In fact, at that level, learning a new language is just a matter of getting used to a new syntax. Most of the concepts sound familiar to you.

Here are my 3 absolute favorite books. I’ve read each of these twice! Why only 3 books? I could list 30 books here, and chances are you wouldn’t read any of them because there would be too many too chose from! Less is more.

I handpicked the top 3 that I loved the most amongst all the other books I’ve read. And none of these books teach you the C# programming language. I’m assuming you’ve read one or more books on the topic or watched my courses. So, these books are “supplementary” materials to make you a better developer.

Agile Principles, Patterns and Practices (Robert C Martin)

Agile Principles, Patterns and PracticesRobert C Martin (Uncle Bob) is my guru! I’ve learned a lot from his books and videos. I’ve read most of his books twice and watched most of his videos. Even though the title has the word “agile”, the books is not much about agile. It’s mostly about SOLID principles of object-orientation and design patterns. The book is simple and easy to read and examples make sense.

 

Applying UML and Patterns (Craig Larman) 

51gVLEtrCNL._SX398_BO1,204,203,200_Shall I say the best or the only OOP book worth reading? There are many books on the topic but most of them are boring to death. Craig teaches you OOP in a real-world scenario. He walks you through the requirements of two applications and uses iterative development to build them. The book is around 900 pages, but I promise it’s worth reading. Prior to reading this book, I had read many books about OOP and used it for years. When I read this book, I realized I knew nothing about OOP!

Art of Unit Testing (Roy Osherove)

51VAZ9BwcvL._SX397_BO1,204,203,200_Great book if you want to learn all about automated testing and test-driven development. Roy has been very well respected in the area and he has been running workshops around the world teaching automated testing to C# developers. The book is just over 200 pages and you can easily read it.

 

 

What are your favorite C# books? Please drop a comment and share!

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What text books tell you about inheritance in OOP is wrong

I received an email from one of my students asking for a design advice. He is working on an application used by a sports team manager, where they can create, edit and delete players, teams and coaches. So, the design that my student had in mind is something like this UML diagram:

UML

So, a SoccerPlayer is a Player, which in turn is a Person. This is what many text books on object-orientation programming teach you:

When you can express the association between two concepts using IS-A, that’s an inheritance!

Many of these books claim that by creating these abstractions, in this example Person, you can “re-use” them in other projects. However, in 15 years of my professional experience, I have hardly seen many (or any) examples of such abstractions being used across projects in a useful manner.

At one point, I started creating a class library of these abstractions that I could share across projects. After a little while, that class library ended up being bloated with lots of unrelated abstractions and versioning it across different projects ended up being a nightmare. If you have done this before, you may relate to this story.

An extreme example of inheritance

Once I was engaged as a consultant for a greenfield project, and in the introductory session, the data architect of the project walked me through tons of UML class diagrams (more than 50 pages). Every class was inheriting from another class and eventually, they all led to a class that was called “Thing”! No joke!

Interestingly, there was not even one behavioural diagram expressing the behaviour of the system. So this huge inheritance hierarchy was only for inheriting “attributes”, not behaviour, which is what inheritance is all about!

Guess what? After spending more than a year and half a million dollars on that project, the company decided to shutdown the project. Nothing was produced, not even a beta version! They engaged various consultants and teams and no one could implement an application with such a large complex model.

Does this story sound familiar to you?

Don’t model the universe!

When building software, you should design a domain model for an application, based on the needs of that application and not the reality. The use cases that the application is going to fulfil will determine the angle at which you should look at the domain model.

In the real world, a SoccerPlayer is a Player and a Player is a Person. But just because you can express such relationship in English, doesn’t mean you should add such association between your classes. You’ll end up polluting your model with lots of unnecessary complexity that serves no purpose.

Inheritance increases coupling in your design

So what is wrong with inheritance? Well, before explaining, let me clarify something. I’m not anti-inheritance! Inheritance, just like every thing else, has its uses. When you use it in the right context, it works for you. But if you abuse it, it leads to increased complexity in your applications.

Inheritance creates tight coupling between your classes. If class Child derives from Parent, it is tightly coupled to the Parent. If you make any changes to the Parent, you may have to modify the Child as well. Or at a minimum, you need to re-compile it and re-deploy the assembly in which it is defined.

Now, when you have a small hierarchy with a few classes, that is not an issue. But as your hierarchy grows, the impact of change grows in your application. The further on top of the hierarchy you make changes, the more classes may be affected or at least need to be re-compiled.

When to use inheritance

So, when should you use inheritance? When re-using behaviour and potentially overriding it, which leads to polymorphism. But even then,  you could use composition to achieve the same thing with less coupling in your design. The famous saying favour composition over inheritance explains that. I’ll write a separate post on this topic soon.

Does this sound too abstract for you? Ok, continue reading for some simple pragmatic tips.

When not to use inheritance

I love theory in practice. If you’re the same, let me give you my super simple and pragmatic advice by showing you some code. If you have one or more of the following symptoms in your code, you probably don’t need inheritance. You can collapse your hierarchy, reduce the coupling and simplify your design.

When you have hollow classes

Do you have classes like this in your design?

public class Person 
{
     public int Id { get; set; }
     public string Name { get; set; }
}

public class Player : Person 
{
}

In case you’re not familiar with the C# syntax, here we have a Person class with two attributes: Id and Name. The Player class derives from Person.

The Player class here is what I call a hollow class. It serves no purpose. We could simply use the Person and reduce complexity in our design.

Here is a variation of this example:

public class Person 
{
}

public class Player : Person 
{
     public int Id { get; set; }
     public string Name { get; set; }
}

In this example, Person is a hollow class and is useless.

Here is the important part I want you to pay attention to: in the domain of this application, what matters is the Id and Name of each Player. The fact that a Player is a Person in the real-world does not matter in this application. And that’s what I meant by not modelling your applications based on the reality. Instead, you should model based on what the application needs and how it is going to behave.

When your inheritance hierarchy is all about attributes

Look at this example:

public class Person 
{
     public int Id { get; set; }
     public string Name { get; set; }
}

public class Player : Person 
{
     public byte Number { get; set; }
}

public class Coach : Person 
{
     public byte YearsOfExperience { get; set;
}

The argument behind this design is that we’re re-using the Id and Name properties in the Player and Coach classes. I used to have many classes like this before! The extreme example I mentioned earlier was exactly like this.

But re-using attributes is a poor way of thinking about inheritance. How much effort would be to duplicate these attributes in the derived classes? It would take only a second to copy/paste them. You may say but duplicating code is bad. Yes, but not always! It is bad if you’re going to change it and have to make the change in multiple places. But how often would these attributes “change”? Not often or never! How about the logic? Logic, algorithm and behaviour can change often.

That’s the reason that you should reserve inheritance for re-using behaviour, not attributes. But even then, you can use would better use composition.

Let’s sum it up

So, remember, inheritance increases coupling between your classes. Reserve it for situations when you want to implement polymorphism, not just for re-using code, especially attributes. If you have one or more of the following symptoms in your design, you probably don’t need inheritance:

  • Hollow classes that have no members
  • Base classes that include only attributes. They don’t have any methods for the derived classes to re-use or more importantly, override.

If you see this in your design, you can achieve a simpler, cleaner and more maintainable design by collapsing your inheritance hierarchy.

So, keep it simple!

 

If you enjoyed this post, please share it and leave your comments below.

 

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